Coating materials which cure by two independent processes are referred to generally as dual-cure systems. Customarily the binder components present possess different functional groups, which under suitable conditions, generally independently of one another, react with one another and, in so doing, crosslink. Customary prior-art dual-cure systems possess radiation-curing and thermally curing groups, with particularly advantageous properties being obtained when using isocyanate groups and hydroxyl groups as thermally crosslinking functions.
EP-A 0 928 800 describes a dual-cure system which comprises a crosslinker that contains both radiation-curing acrylate groups and free isocyanate groups which can be cured thermally using suitable binders, e.g. OH-containing binders. Since NCO groups and OH groups react with one another even at room temperature, the aforementioned coating system can be employed only as a two component system in which NCO-containing and NCO-reactive constituents are mixed with one another shortly before or during the coating operation. The disadvantage of a very short processing time for these systems can be alleviated by blocking the free NCO groups. The combination of such radiation-curing and heat-curing systems containing blocked isocyanate groups is described for example in EP-A-126 359, WO-A 01/42329 or U.S. Pat. No. 4,961,960.
In the case of the majority of prior-art blocked polyisocyanates the blocking agents present are eliminated during the crosslinking reaction and then released. On the one hand this adversely affects the VOC content of the coating systems; on the other, eliminated blocking agent remains in the paint film, thereby adversely affecting the properties of the coating. As a result, scratch resistance and acid stability of one-component (1K) paint films are generally significantly poorer than in the case of two-component (2K) polyurethane coatings (e.g. T. Engbert, E. König, E. Jürgens, Farbe & Lack, Curt R. Vincentz Verlag, Hanover October 1995). The elimination of the blocking agent and its gaseous escape from the paint film may lead, furthermore, to blistering in the paint. Subsequent incineration of the emitted blocking agent may possibly be necessary.
For particularly low crosslinking temperatures in the range from 90 to 120° C., use has been made more recently of diethyl malonate blocked isocyanates (e.g. EP-A 0 947 531). In contrast to blocking with, for example, N-heterocyclic compounds, such as caprolactam or butanone oxime, in this case the blocking agent is not completely eliminated; instead, in the course of curing, there is a transesterification on the diethyl malonate, with elimination of ethanol. A disadvantage, however, is that, because of the labile ester bond, such systems are extremely susceptible to acid exposure, and so the application possibilities of these products are limited.
The object on which the present invention is based was therefore to provide new blocked polyisocyanates which cure both thermally and photochemically and at the same time this process proceeds without emissions.